Four patients had an immediate reaction: Two had hives, one had redness at the injection site, and the other had nausea.

A total of 292 (65.62%) patients receiving IV contrast documented a delayed side effect, and 80 (68.38%) in the control group reported delayed side effects. There was no statistically significant difference between the groups. Considering the gastrointestinal side effects alone, however, revealed that 153 (34.38%) patients reported a side effect following contrast, while 56 (47.86%) reported one without having received IV contrast. This was statistically significant (p = 0.0097).

The table shows the frequency of delayed side effects in patients with a history of asthma, allergy, and previous drug reactions, as well as patients receiving chemotherapy and steroids. Patients with a previous history of asthma, allergy, and drug reaction show significantly more chance of developing a delayed allergic reaction. Patients on chemotherapy (significant, p = 0.0001) and steroids (not significant, p = 0.3807) showed a lower occurrence of delayed side effects.

With regard to various different types of contrast, no significant differences were seen. Patients receiving iomeprol had slightly more GI complaints, and iohexol showed a slight advantage in skin reactions (see graph). Women suffered more allergic reactions, with 41 (65%), 90 (59%), and 50 (66%) documenting skin, gastrointestinal, and general delayed reactions, respectively, compared with men, who had a reaction in 22 (35%), 63 (41%), and 26 (34%) cases, respectively.

The parent molecule from which the contrast agents are derived is benzene. Iodine is the element used in contrast media because it possesses high contrast density, firm binding to the benzene molecule, and low toxicity, which are three properties essential for the production of contrast media.

Because of their chemical properties, contrast media are usually viscous and have greater osmolality (more molecules per kilogram of water) than blood, plasma, and cerebrospinal fluid. Viscosity and osmolality play a part in the development of contrast reactions. High-osmolality contrast media (HOCM) are five to eight times more osmolar than plasma. Low-osmolality contrast media (LOCM) are two to three times more osmolar than plasma, and iso-osmolar contrast media has the same osmolality as plasma. Iso-osmolar contrast media are increasingly used. The incidence of mild and moderate contrast reactions is higher for HOCM (6% to 8%) than for LOCM (0.2%), but the incidence of severe reactions remains similar.